How do you develop reusable function blocks in PLC programming?

Function block development: define inputs (parameters, references), outputs (results, status), internal variables (state storage), and logic. Design principles: single responsibility (one block = one function), well-documented interfaces, configurable parameters, error handling with status outputs, and version control. Examples: motor control FB with start/stop/fault logic, PID FB with auto-tune capability, valve control FB with position feedback. Test thoroughly with edge cases. Maintain library of proven FBs. Using FBs reduces development time, ensures consistency, simplifies troubleshooting, and enables modular design.